The invention relates to a method for reducing the false alarm rate on moving targets in radar images of two or multi-channel MTI radar devices of the type using space time adaptive processing.
This application claims the priority of 102 07465.8, filed Feb. 22, 2002, the disclosure of which is expressly incorporated by reference herein.
Raw radar data consists of a mixture of signals, e.g. echoes of stationary and moving objects and interferences, e.g. broadband and narrowband noise. When evaluating the radar data generally the interferences are supposed to be suppressed so that the signals of interest can be detected well.
For the extraction of moving objects it is standard practice to employ so-called MTI (moving target indication) signal processing. Apart from identification and recognition through range Doppler frequency analysis, parameter estimation such as for distance, angle of view and velocity of moving targets is part of this signal processing method. With this method the echo of the moving target is essentially disrupted by the receiver noise.
In the case of air-ground radar devices or ground-air radar operating modes, which are characterized by a relatively clear spectral broadening of the direction of maximum radiation of the antenna (main antenna lobe), additionally echoes of stationary objects occur, which are erroneously interpreted as moving targets. These false target alarms occur mainly in the range Doppler frequency range of the direction of maximum radiation of the antenna (main antenna lobe) and with low main/side lobe distance of the antenna also in the antenna side lobe range.
FIG. 1 shows an MTI radar image with detected fixed and moving targets, which are clearly visibly dispersed across the entire radar image. They are detections in the main antenna lobe area 1, in the side lobe area 2 and in the antenna lobe free area 3. In the case of a large number of detections, echoes of fixed targets are involved, which lead to false alarms that have to be recognized and eliminated.
For suppressing echoes of fixed targets generally the so-called STAP (space time adaptive processing) method is applied, which we know e.g. from German Reference DE 100 12 411 A1 or DE 100 35 530 A1. The STAP method takes advantage of the deterministic amplitude and phase correlations between adjacent antenna channels of a coherent pulse Doppler radar device. These correlations are adaptively estimated from the raw radar data. The STAP method however is generally only applied in the main antenna lobe area.
In conventional MTI signal processing, the detection of moving targets occurs within the antenna lobe free region through CFAR (constant false alarm rate) algorithms. With CFAR the amplitude amount or the power value of a potential target is compared to the average amplitude amount or the average power value of its environment. When the amplitude amount or the power ratio exceeds a specified threshold value, target alarms are issued. Due to the Doppler processing step they are allocated to moving targets. The method is normally not applied in the main antenna or side antenna lobe areas. The non-homogeneity of the environment results in an extreme number of detections, which would largely have to be attributed to non-moving targets.
It is the object of the invention to provide a method, which enables a considerable reduction in the false alarm rate in radar, images compared to the state of the art.
The present invention involves:
providing an STAP filter with visible side antenna lobe in the radar image for the suppression of fixed targets in the side antenna lobe,
comparing the amplitude amount or power value of each range Doppler frequency cell with a threshold that can be specified in constant terms across all range Doppler frequency cells and summary of the resulting alarms, wherein only those alarms are clustered whose amplitude amounts or power values are above the specified threshold, wherein first a summary of alarms that is directed in the direction of the Doppler frequency into Doppler frequency clusters occurs based on their minimal distance to each other and subsequently a summary of Doppler frequency clusters that is directed in the distance direction into range Doppler frequency clusters occurs based on their minimal distance to each other,
comparing of a target function with a test function, wherein the test function is an intersecting function of a two-dimensional characteristic function of the amplitude and phase progressions of the main antenna lobe as a function of the Doppler frequency and the target function is a characteristic function, calculated for each alarm, of the amplitude and phase progressions of the respective target as a function of the Doppler frequency on the Doppler frequency position of the respective target, and
determining selection criteria from suitable combined calculation methods that are applied to the test and target functions for the purpose of identifying a respective target alarm as a moving target or fixed target.
One benefit of the method pursuant to the present invention is a reduction in false claim rate when utilizing the multi-channel feature of the coherent radar system, i.e. the decision to declare a fixed target or a disturbance as a moving target, is reduced considerably. Additionally, the present method achieves an improved reduction in the false alarm rate even with a poor main lobe/side lobe distance of the antenna because the method pursuant to the invention permits the adaptive estimation of the side antenna lobe progression.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.